题目

线性表是由个元素组成的有限序列。当n=0时，为。 答案：【1】n (n>=0)【2】空表 1.They heard a message from a spacecraft.(用just改为现在完成时态形式) They        just        a message from a spacecraft. 2.Have you got a letter from Lucy?(改为同义句) Have you                Lucy? 3.To understand the problem is very difficult.(用it作为形式主语)                very difficult        understand the problem. 4.I came here two days ago.(改为同义句) I                here         two days. 5.Her mother has been to Hong Kong several times.(就画线部分提问)                         has her mother been to Hong Kong?